An 18" Mossberg or Remy would easily suit your needs. I'd go with 12ga and consider packing along some 12ga (marine) flares with your buckshot and/or slugs - - you just never know.
Here's a link to ATI and some of their accessories.
http://www.atigunstocks.com/product-rem870.html
For ease of carry, over the shoulder, in a scabberd, or in a backpack I'd seriously consider the "pistol-grip top-folder" or the "pistol grip telescoping" stock.